What you are thinking is polymer, Cynic, is the outer barrel. The outer barrel is made from polymer to increase the the durability and reliability. The gun will cycle better if it is rubbing against a polymer outer barrel, as opposed to a metal one.

Whoops, I meant to say the outer barrel. Sorry about the confusion. Is there a replacement after part to cover the outer barrel to metal? I prefer the metal feel and sound even if its not as efficient.

There have been some aftermarket products sold that you can switch your outer polymer barrel to a metal outer barrel. KWA even sells a metal outer barrel for their MK series. Keep in mind, these will reduce performance (perhaps not significantly) but will increase and accelerate the amount of wear and tear on your gun. I don't see any harm in replacing an outer barrel, just be sure you know what you are doing.

a lot of the HK45 that have been sold at a local airsoft shop have come back with the infamous cracked outer barrel that so many are having..including me. i have found that it cracks right in the center, top of chamber, on the seam. now i do know for a fact that the plactic stock outer barrels from the USP KP45 and USP TACTICAL drop right into the HK45 and work flawlessly..